So until 2-3 days ago Sonarr/Transmission were working in harmony on my new setup - but now there’s no connection, and I can’t manually/automatically download anything. I fiddled with ports and no joy (have since restored to default/reinstalled Sonarr)
Transmission is doing or returning something unexpected, can you open the web interface in a browser? Make sure you’re using the WebUI port in Sonarr and not the port Transmissions uses for BT connections.
((Trace logs)) from Sonarr may show something, but it looks like it’s failing before Sonarr gets a response.